1. Field of the Invention
The present invention relates mainly to a stroboscope circuit capable of controlling a dimmer operation of an electronic flashlight and, more particularly, to a series-type automated stroboscope dimmer circuit which cuts off a current flowing through a flash lamp when an intensity of an exposure light reaches a predetermined value.
2. Brief Description of the Prior Art
A circuit, for instance, shown in FIG. 3 has been proposed up to now as an electric current cut-off type automated stroboscope dimmer circuit.
In FIG. 3, a numeric character 1 stands for an electronic flash lamp, for instance, a xenon tube while 2 stands for an insulated gate bipolar transistor (referred to as "IGBT" hereinafter) which is connected in series with respect to the xenon tube 1. Q1 and Q2 are pnp-type transistors which are connected in series to each other. A gate electrode of the IGBT 2 is connected to an emitter-sided electrode of the transistor Q2 and substantially connected also to a collector electrode of the transistor Q1. On the other hand, a base electrode of the transistor Q1 is connected to a collector-sided electrode of a pnp-type transistor Q3.
Another numeric sign 3 stands for a dimmer circuit, 4 stands for a power supply terminal for receiving a driving voltage to be supplied to the xenon tube 1, 5 stands for an input terminal for receiving a control voltage signal VB and 6 stands for another input terminal for receiving a synchronous signal (a timing signal) SY. Herein R2 to R4, R6 and R8 to R10 stand for resistors, respectively.
Aforesaid circuit employs the IGBT 2 as a switching device which cuts off the current. When the flashlight illuminated from the discharging xenon tube 1 extends to a certain intensity, a dimmer action is performed by switching the IGBT 2 from a conductive status to a cut-off status thereby stopping the current flowing through the xenon tube 1.
Namely, when the synchronous signal SY is switched from an H (high level) status to an L (low level) status first, the transistor Q1 turns on and the control signal voltage (the gate voltage) VB is applied to the gate electrode of the IGBT 2. Accordingly, the IGBT 2 turns on to fire the xenon tube 1, by which a flashlight illumination is initiated.
When the intensity of the flashlight emitted from the xenon lamp 1 reaches a desired value, an output signal A transmitted from the dimmer circuit 3 turns from an H status into an L status, which turns on the transistor Q2. The turning on of the transistor Q2 turns off the IGBT 2 as well as turns the transistor Q3 into a conductive status, the latter of which turns off the transistor Q1 on the contrary. As a result, the gate voltage supplied to a gate electrode of the IGBT 2 is terminated.
In such an automated stroboscope circuit as mentioned above, the transistors Q2 and Q3 are simultaneously turned on to turn off transistor Q1 during dimmer operation. However, such a circuit configuration is apt to induce some chattering phenomena in switching operation, which encounters with a big problem that causes sometimes fatal breakdowns in the IGBT 2.
On the other hand, aforesaid circuit configuration requires simultaneously three pieces of transistors, which proposes another problem that the circuit requires many pieces of expensive electronic components called semiconductor devices.